Yet as the first Turkish voters cast their ballots across the country on Sunday, the polls showed a surprisingly close race with Yes leading at around 51 per cent.

Turnout is expected to be high with more than 80 per cent of Turkey’s 55 million voters likely to cast ballots in the historic referendum. Results are expected at around 11pm Turkish time (9pm UK time).

A voter prepares to cast a ballot inside a polling station in IstanbulCredit: AP Photo/Emrah Gurel

A Yes vote would scrap the role of prime minister and give Mr Erdoğan powers to appoint senior judges, dissolve parliament, declare a state of emergency and in some cases make law by decree.

Under the new system, Mr Erdoğan could theoretically remain in power until 2029. He first took office as prime minister in 2003 and has dominated Turkish politics ever since.

Mr Erdoğan argues that the new system is necessary for a strong government that can provide stability as Turkey faces terrorism, spillover from the Syrian war and internal threats like last summer’s coup attempt.

Supporters of Turkey's President Erdogan wave national flags during a rally for the referendumCredit: REUTERS/Umit Bektas

For much of its history Turkey has had unstable coalition governments, changing roughly every two years, and Mr Erdoğan argues that the new presidential system will provide a strong platform for growth and reform.

“Tomorrow is very important, you must absolutely go to the polls,” Mr Erdoğan said at his final campaign rally on Saturday. “Don’t forget that the vote is our honour.”

Opponents argue the system is a major lurch towards dictatorship, with too much power concentrated in the hands of the president and insufficient checks and balances to constrain him from authoritarianism.

“These constitutional changes pose a huge threat to human rights, the rule of law, and the country’s democratic future,” said Human Rights Watch.

If the referendum proposals go through they would be the most dramatic changes to Turkey’s political system since opposition parties were first allowed to compete for votes in 1946.

Mr Erdoğan has staked his own credibility on a Yes vote, putting himself at the centre of the campaign. Every Turkish city and town is covered in thousands of banners bearing his image and he has held daily rallies since January urging a Yes vote.

His opponents have struggled to be heard in state media and pro-Erdoğan private media outlets and opposition politicians from the Kurdish-rooted HDP party have been arrested.

Mr Erdoğan has equated leaders of the No campaign with terrorism and suggested they might have been in league with the leaders of a failed coup attempt launched against him last July.

The referendum campaign triggered an explosive row with the EU after Holland and Germany refused to allow Turkish ministers to hold rallies with Turkish expatriates in their countries. Mr Erdoğan accused the EU of being "Nazis" in response.

In Kasımpaşa, the working class neighbourhood of Istanbul where Mr Erdoğan was raised, supporters said they were excited to cast their votes in support of the president. Yes banners hung among the laundry draped off washing lines.

“He’s one of us,” said Rukiye Karagolu, who runs a small corner shop and has known Mr Erdoğan for decades. “On Fridays he would come here and buy treats for the children and now he’s a world leader. But he didn’t change at all. He’s exactly the same as he was.”

But in Kadikoy, one of Istanbul’s most liberal neighbourhoods, No voters said they were anxious about the results. “We collapsed the Sultan’s system and we changed to a republic and now we want to go back?” said Mehmet Gacgili, a 65-year-old retired sailor.